The mechanism for the increase in plasma renin activity (PRA) due to sinoaortic denervation and cervical vagotomy was examined in intact, renal denervated, and renal denervated, adrenalectomized dogs maintained on a high salt diet. These data suggest that the increase in PRA after sinoaortic denervation is probably due to both increased sympathetic discharge to the kidney and catecholamines released from the adrenal medulla, whereas the increase in PRA after cervical vagotomy is mediated largely by increased sympathetic discharge to the kidney.